1. Field of the Invention
The present invention relates to an optical encoder for use in combination with, for example a rotary shaft of a motor or the like, and more specifically to an optical encoder using a code plate formed of resin material.
2. Description of the Related Art
As a code plate of an optical encoder requiring high accuracy, one made of clear glass, which is provided with a pattern by chrome deposition and etching, has been conventionally utilized. In recent years, however, due to the improvement of the molding and transferring technology, an optical resin which is colorless and transparent is much more likely to be used instead of glass materials. The main reason for this is that in general resin material, compared to glass, are excellent in mass productivity because of ease of molding and have an advantage in cost for manufacturing. There are many different kinds of well-known constructions of code plates made of transparent resin. One of examples of such constructions is illustrated in FIG. 1.
As shown in FIG. 1, a code pattern of the resin-made code plate includes flat portions serving as translucent sections which directly transmit the light supplied from a light source (not shown) and portions with a series of V-shaped grooves, which serve as non-translucent sections (reflecting sections). Each V-shaped groove of the grooved portions is formed to have an oblique angle causing total reflection of the light incident from the inner side of the code plate. The resin-made code plate with such a code pattern is usable also in an optical encoder of a type which detects either a transmitted or reflected light by a detecting unit. In case that a transmitted light is detected by the detecting unit, the transmitted light forms so-called “bright sections”, and so-called “dark sections” are produced correspondingly to the portions with a series of V-shaped grooves (reflecting sections). On the contrary, in case that a reflected light is detected by the detecting unit, the reflected light creates so-called “bright sections”, and so-called “dark sections” are formed correspondingly to the translucent sections.
An infrared LED is largely utilized as a light source which emits light for detection. According to this, used as a light-receiving element for detection is infrared light having wavelengths between about 800 nm and about 1000 nm and also having high sensitivity. Used as a transparent resin in general is polymethyl methacrylate (PMMA), polycarbonate (PC) or the like. These materials, as described later, have high spectral transmittance in a broad wavelength range extending from blue light to infrared light. The materials are substantially transparent and colorless, and also excellent in moldability and transferability. Because of such material properties, these materials are extensively used not only for resin code plates but also for other optical elements, such as optical discs and lenses of CDs and DVDs.
However, resins, such as polymethyl methacrylate and polycarbonate, which have been conventionally used for making code plates, have a great disadvantage in heat resistance and oil resistance (resistance against oily substances). This disadvantage is not negligible since the optical encoder is most typically applied in the periphery of the rotary shaft of an industrial motor, which is liable to reach a considerably high temperature. In such a place, there is fear that the optical encoder contacts high-temperature steam and cutting oil. If a code plate made of polymethyl methacrylate or polycarbonate is utilized in such a harsh environment, this produces problems that the high-temperature steam and the cutting oil cause deterioration and white turbidity of the surface phase the code plate, that the surface of the code plate is melted to alter the shape of surface of the V-shaped groove, which makes it impossible to obtain a satisfactory light-and-shade contrast, and the like.
Disclosed examples of an optical encoder using a resin-made code plate includes JP 7-270185A, JP 10-170308A, JP 10-239108A, and JP 11-201779A. JP 7-270185A describes a rotating disk (code plate) formed by bonding two transparent film disks whose base materials are a plastic resin to each other, applying a sensitive material onto one side thereof to form a slit side having a plurality of slits in the circumferential direction, and forming a transparent side made of the same material as the slit side on the other side, in order to averting the deformation of the rotating disk (code plate).
Disclosed in JP 10-170308A is a rotary encoder using a code plate formed by providing a plurality of recessed and projecting parts onto a disk made of a light-transmitting synthetic resin and having a construction in which the recessed parts are filled up with a substance which does not transmit light. JP 10-239108A describes a code plate whose converting portion (code pattern), which is composed of a line of lens elements, is formed by plastic-molding. JP 11-201779A discloses an acrylic resin-made code plate sectioned into a first and a second type of strip-shaped regions to generate a first and a second type of output light having mutually different light paths, in which at least either one of the first or second type of region has a light path changing function.
Although each of the above-described examples of prior art uses plastic as a construction material of the code plate, none of them teaches using a colored resin material. This means that, on the level of prior art, there has not been awareness that the code plate may be formed of a colored material as long as it has sufficient spectral transmittance with respect to the wavelength of the detection light, and that it has been assumed that material used for the code plate of an optical encoder must be transparent and colorless. Such fixed ideas have hindered free choice of material having great resistance against the high-temperature steam, cutting oil, etc.
As for JP 7-270185A, although a countermeasure against deformation is taken, the idea disclosed in the document is not based on the concept that the code plate may be colored if having sufficient spectral transmittance with respect to the wavelength of the detection light. Thus, the invention disclosed in the document intends to solve the problems by forming the code plate with the construction in which two film disks are bonded to each other. Such a bonding construction, however, is disadvantageous in terms of costs, and moreover the bonded portion would possibly come unstuck.